Endoscopes are often used to visualize a surgical site. Mechanical damage to the endoscope can easily occur if there is undesired contact with a surgical instrument. In order to prevent this, a protective sleeve with a cover glass can be provided, with the result that only the protective sleeve, and not the endoscope, is damaged.
As the surgical site is often filled with a saline solution, there is refraction at the transition between the saline solution and the cover glass of the protective sleeve, which leads to deterioration of the imaging properties of the endoscope. In particular, e.g. the maximum angle of view of the imaging lens system can be reduced.